Differential Phase Shift Keyed (DPSK) modulation is used extensively in binary digital data communications. Data are transmitted by periodically shifting the phase of a constant frequency carrier wave. Information is encoded as the phase difference (shift) between adjacent baud intervals and it is possible to have a particular phase shift represent more than one binary digit (bit). For example, the following bit pairs might be represented by the tabulated phase shifts.
______________________________________ BIT PAIR PHASE SHIFT ______________________________________ 11 +45.degree. 0.degree. 10 -45.degree. 90.degree. 00 +135.degree. 180.degree. 01 -135.degree. -90.degree. ______________________________________
Each particular set of phase shifts (e.g., 0.degree., .+-.90.degree., 180.degree.) is called a constellation while the individual phase shifts are referred to as vectors. The above table illustrates two possible constellations for use in a 4-phase DPSK system.
DPSK receivers utilize a reference frequency available in a plurality of phase states equal in number to one-half the number of transmitted phase states. The reference phase states are selected to reside midway between certain adjacent transmitted states. When the transmitted constellation is, for example, (.+-.45.degree., .+-.135.degree.) the only additional reference phase that needs to be generated is a 90.degree. vector (a 0.degree. vector is already available as the incoming signal itself). Such a system is described in U.S. Pat. No. 3,128,343, issued to P. A. Baker on Apr. 7, 1964.
However, when the transmitted constellation is (0.degree., .+-.90.degree., 180.degree.) the use of the 0.degree. and 90.degree. reference phases do not yield to polarity detection alone. It is therefore an object of this invention to avoid magnitude measurements in the demodulation of DPSK signals.
When the transmitted constellation is (0.degree., .+-.90.degree., 180.degree.) reference phases of .+-.45.degree. are a natural choice for multiplication with the incoming signal to achieve demodulation. Indeed, a choice such as this is indicated at page 205 in the book Data Transmission, by Bennett, W. R., and Davey, J. R., copyright 1965, McGraw-Hill, Inc. A pair of phase shifting networks is therefore needed to achieve demodulation. Although polarity detection is sufficient for demodulation of the DPSK signal, the use of multiple phase shift networks is both costly and cumbersome.
Accordingly, it is another object of this invention to minimize the number of phase shifting networks used in generating the reference signals for DPSK detectors.